Adds RemoteDefaultBranch() to git.go that detects the repo's actual
default branch by checking origin/HEAD, then falling back to checking
for origin/master or origin/main.
Updates done.go to use this detection instead of hardcoded "main":
- Line 168: CommitsAhead now uses detected default branch
- Line 173: Error message uses detected branch name
- Line 187: Target branch defaults to detected branch
Fixes repos using 'master' as default branch (pre-2020 repos).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Steve Yegge <steve.yegge@gmail.com>
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